The structure of calmodulin and its interaction with three specific proteins will be examined employing a range of physical techniques. The possible existence of internal molecular flexibility as a factor influencing the properties of calmodulin will be tested for by dynamic measurements of fluorescence anisotropy decay and by static measurements of fluorescence anisotropy, employing specifically located fluorescent probes. The properties of each of the Ca2+ liganded states of calmodulin will be characterized with respect to molecular flexibility, helical content, and spatial extension. The interaction of calmodulin with phosphorylase kinase, tubulin, and myosin kinase will be studied in detail. In all cases the thermodynamic parameters characterizing the binding interaction and any conformational changes accompanying binding will be examined. The molecular mechanism for the regulatory action of calmodulin will be sought in each case.